In so-called comparison microscopes, which are utilized in particular in forensic investigations, two objects are compared with one another in two microscopes located next to one another. The two microscopes possess microscope optics of identical design that are connected to one another via an optical coupling apparatus that is also referred to as an “optical bridge.” The left and right light bundles generated in the two microscope optics are combined via an optical element, usually a prism, arranged in the coupling apparatus. The two object images of the left and the right microscope optics can thus be viewed simultaneously in comparative fashion, selectably next to one another or superimposed onto one another, via an eyepiece and/or a camera.
If the two objects to be viewed in comparative fashion are parts of one and the same specimen, e.g. parts of a torn sheet of paper, the object images generated by the two microscope optics should produce the same color impression on the viewer. For this, the optical element that combines, in the optical bridge, the two light bundles generated by the two microscope optics must possess sufficiently precise color uniformity and, ideally, also a high degree of color fidelity.
“Color uniformity” is to be understood here as the ability of the optical element to influence the spectral energy distributions of the two light bundles passing through the optical element in identical fashion spectrally, i.e. as a function of wavelength. This means that in terms of the light bundles passing through, the optical element in its entirety must exhibit transmittance values that match in the predetermined working wavelength range (if applicable, except for a wavelength-independent constant). In other words, the two light bundles may experience wavelength-dependent changes in their spectral energy distributions as they pass through the optical element, but those changes are the same for both light bundles.
“Color fidelity,” on the other hand, is to be understood as the ability of the optical element to modify the spectral energy distributions of the two light bundles passing through the optical element, in the working wavelength range (if at all), in identical fashion for all wavelengths. This means that in terms of the two light bundles passing through the optical element, the element in its entirety has transmittance values that are constant in the working wavelength range.
In an optical element of the kind described above, combining of the light bundles is usually brought about by means of a beam splitter layer, arranged inside a transparent body, that reflects a portion of the respective light bundle with a predefined reflectance and transmits a portion of the light bundle with a predefined transmittance. The light bundles can be combined in the desired fashion by being suitably directed onto this beam splitter layer and then respectively reflected and transmitted there.
It is problematic in this regard that the reflectance and transmittance of a beam splitter layer of this kind as a rule are not constant within the working wavelength range, but instead vary significantly with the light wavelength. The beam splitter layer thus cannot be manufactured with the desired properties such as color uniformity and color fidelity. The beam splitter layer instead influences the incident light bundle, in accordance with its spectral reflectance and transmittance, in such a way that the spectral energy distributions of the light bundles, which are still identical before entering the optical element, differ so greatly from one another after passing through the optical element that the objects imaged by the light bundles are reproduced in color-distorted fashion. This property of the beam splitter layer (or of the optical element containing the beam splitter) is referred to hereinafter simply as “color error.”
US 2004/0240049 A1 describes a method in which two light bundles are combined via a prism. To achieve improved color fidelity, the entering light bundles are split and recombined by a complex prism geometry. This method is disadvantageous in that troublesome white light interference can occur as a result of the splitting and recombination of the light bundles.
CH 302963 describes a beam splitter that is made up of multiple mirrors whose surfaces are furnished with metallic reflection coatings in order to achieve color uniformity. This minor system is not very compact because of the number of its separate components, and is therefore difficult to integrate into optical devices.
DE 103 56 890 describes a method in which, for a system constituted from a light source, a beam splitter, and a detector, color uniformity is achieved with the use of bandpass filters. This method is disadvantageous in that it is applicable only specifically to the overall system in question, including the light source and detector. In addition, it may prove troublesome in certain applications that spectral light components are filtered out of the usable light by the bandpass filters.